Rolling part and power transmission part

ABSTRACT

There is provided a rolling component formed of a steel material of the medium carbon steel level that provides a rolling life improved to be comparable to that of bearing steel and also provides improved characteristic against surface-cracking, and a power transmission component including the rolling component. To achieve this, the rolling component is formed of steel having an induction hardened portion having a stress intensity factor range associated with tension associated-fatigue crack extension that has a lower limit ΔKth of 6.2 Mpa√m.

RELATED APPLICATION

This application is the U.S. National Phase under 35 U.S.C. Å 371 ofInternational Application No. PCT/JP03106237, filed May 19, 2003, whichin turn claims the benefit of Japanese Application No. 2002-149428,filed May 23, 2002, the disclosures of which Applications areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to rolling components formed of inductionhardened steel material, and experiencing repeated tensile stress andused for example under a severe condition for lubrication and/or acondition accompanied by slippery, and power transmission componentsincluding the rolling components.

BACKGROUND ART

Rolling bearings and other geometrically simple rolling components areformed of SUJ2 or similar bearing steels providing long rolling contactfatigue life. Bearing steel, however, is poor in workability andunsuitable for rolling components having complicated geometries. Incontrast, S53C and similar medium carbon steels have satisfactoryworkability and are suitable for rolling components having complicatedgeometries. Typically, medium carbon steel is worked into a complicatedgeometry and then has a rolling portion induction hardened for use.Furthermore, medium carbon steel contains an expensive alloy element ina small amount. It is thus inexpensive and also contributes to savedrare resources.

Rolling components having complicated geometries, however, often receivenot only a simple rolling load at the rolling portion. In addition torolling, there are also slippery and repeated tensile stress other thanrolling superimposed. As such, the rolling portion is prone to cracking.This invites early propagation of cracking and can result in fataldamage. This is considered to be attributed to that medium carbon steelhas a shorter rolling contact fatigue life than bearing steel.

Recently, as energy conservation and miniaturization are pursued,rolling components increasingly tend to be used under severer conditionsthan before. Bearing steel, poor in workability, has its limitation inproviding long life with productivity and cost considered, and thereexists an increasing demand for a rolling component produced from asource material provided by a steel material corresponding toconventional medium carbon steel with its inexpensive alloy componentsC, Si and Mn modified in content. More specifically, there exists ademand for the following items (1) and (2):

(1) As seen at the medium carbon steel level, the induction hardenedportion has a rolling contact fatigue life improved to be comparable tothat of bearing steel; and

(2) As seen at the medium carbon steel level, the induction hardenedportion has increased surface cracking resistance.

Item (1) is effective in improving reliability against rolling fatigueand item (2) is effective in reducing surface cracking attributed toslippery.

DISCLOSURE OF THE INVENTION

The present invention contemplates a rolling component formed of steelmaterial of the medium carbon steel level that provides a rolling lifeimproved to be comparable to that of bearing steel and has an improvedsurface cracking resistance characteristic, and a power transmissioncomponent including the rolling component.

The present rolling component is formed of steel having an inductionhardened portion allowing a stress intensity factor range associatedwith tension associated-fatigue crack extension that has a range with alower limit ΔKth of at least 6.2 MPa√m. The stress intensity factorrange with the lower limit of at least 6.2 MPa√m allows lager resistanceagainst fatigue cracking caused and progressing as tensile stress isrepeatedly exerted than conventional material (or S53C). Conventionally,there has not been a case noting that a rolling component is formed ofsteel material having an induction hardened portion required to havelower limit ΔKth of at least a prescribed value. Furthermore there is nodisclosure of medium carbon steel S53C for a rolling component asconventional that has an induction hardened portion with the lower limitΔKth as above.

Ensuring the above lower limit ΔKth allows a rolling portion to have asurface more resistant to cracking and propagation thereof when it rollsas well as it slides and repeated tensile stress is superimposed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a third generation hub unit including awheel bearing and a constant velocity joint united together that employsthe present rolling component.

FIG. 2 is a schematic view of a fourth generation hub unit including awheel bearing and a constant velocity joint united together that employsthe present rolling component.

FIG. 3 represents a relationship between measured and expected values ofrolling fatigue life L₁₀ associated with rolling in a first example.

FIG. 4 shows a specimen used in a measurement performed in a fatiguecrack extension test in a second example.

FIG. 5 represents a relationship between a correction factor F_(I)(a/W)and α(=a/W) in an expression (4) calculated in the fatigue crackextension test to obtain a stress intensity factor.

FIG. 6 illustrates how stress intensity factor range's lower limit isobtained in a relationship between crack extension rate da/dN and stressintensity factor range ΔK_(I) in the fatigue crack extension test.

FIG. 7 shows a relationship between measured and expected values ofrolling fatigue life L₁₀ associated with rolling in the second example.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter with reference to examples the present invention will morespecifically be described. FIGS. 1 and 2 show hub units employing thepresent rolling component. FIG. 1 schematically shows a third generationhub unit (H/U) corresponding to a hub joint including a wheel bearing 6and a constant velocity joint united together. FIG. 2 schematicallyshows wheel bearing 6 including a forth generation H/U further evolvedfrom the third generation H/U. In the FIG. 1 H/U one inner ring race 2is integral with a hub wheel 4 and the other inner ring race 5 iscrimped to hub wheel 4. An outer ring 3 is fixed directly to a knuckle.In the third generation H/U constant velocity joint 1 is an independentcomponent.

By contrast the FIG. 2 forth generation H/U has a more compactstructure. While one inner ring race 5 is integral with hub wheel 4,which feature is the same as the third generation, the other inner ringrace is integral with a joint outer ring 3. Accordingly that portion isrequired to have (I) a rolling contact fatigue life as the portion actsas a bearing race portion and (II) a slippery accompanied-rolling,swinging life as the portion acts as a joint portion.

EXAMPLE 1

As shown in Table 1, steels A1-A9 falling within the present invention'scomposition range are used as examples of the present invention, andsteels B1-B10 departing from the present invention's composition rangeare used as comparative examples. As indicated under the table as anote, comparative example B1 is a conventional material S53C and B10 isbearing steel SUJ2.

TABLE 1 Alloy Composition (wt %) Type No. C Si Mn Cr Note Present A10.56 0.82 0.83 * Invention's A2 0.60 0.80 0.60 * Examples A3 0.53 0.620.98 * A4 0.63 0.62 0.60 * A5 0.52 1.16 0.74 * A6 0.64 0.83 0.60 * A70.55 0.81 0.60 * A8 0.58 1.00 0.80 * A9 0.61 0.88 0.72 * Comparative B10.53 0.20 0.85 * S53C Examples B2 0.53 1.00 0.31 * B3 0.55 1.00 0.30 *B4 0.63 0.10 0.58 * B5 0.55 0.11 0.60 * B6 0.53 0.38 0.25 * B7 0.53 0.200.25 * B8 0.55 0.20 0.75 * B9 0.45 0.80 0.80 *  B10 1.00 0.25 0.35 1.5SUJ2 * 0.2 to 0.3 wt % thereof contained

(1) Rolling Contact Fatigue Test

As has been described previously, medium carbon steel has a shorterrolling contact fatigue life than bearing steel. When use under severeconditions expected in future is considered, it is desirable that mediumcarbon steel have a rolling contact fatigue life comparable to that ofbearing steel. The specimens are induction hardened to allow a hardenedlayer to have a depth of approximately 2 mm. In this test, the number oftests N is 15, and rolling contact fatigue life is estimated by L₁₀ (10%life). The rolling contact fatigue test is conducted with the followingconditions:

dimension of specimen: 12 mm in outer diameter and 22 mm in length

dimension of counterpart steel ball: 19.05 mm in diameter

maximum contact stress Pmax: 5.88 GPa

load rate: 46,240 cycles/min.

lubricant: turbine oil VG68

(2) Rolling and Sliding Fatigue Test

Needle bearings' counter bearings, constant velocity joints, ball screwsand other similar parts in rolling portions roll and in addition slide.Accordingly they are required not only to have long life as they simplyroll but also as they slide. A rolling and sliding fatigue test is a2-cylinder test conducted to estimate a life of a material as it rollsas well as slides. The specimens are induction hardened to allow ahardened layer to have a depth of approximately 2 mm. The test isconducted with the following conditions:

piece to be tested: 40 mm in outer diameter by 12 mm in width, withoutan outer diameter having the other principal curvature (straight)

counterpart test piece: formed of bearing steel SUJ2 and having 40 mm inouter diameter by 12 mm in width, with an outer diameter having theother principal curvature of 60 mm,

maximum contact stress Pmax: 3.5 GPa

rotation rate: 1,800 rpm for piece to be tested and 2,000 rpm forcounterpart test piece

lubricant: turbine oil VG46

(3) Test Result

Table 2 shows a result of the rolling contact fatigue test and that ofthe rolling and sliding fatigue test. As measured, conventional mediumcarbon steel S53C (comparative example B1) has a rolling contact fatiguelife L₁₀ of 2,630×10⁴ and bearing steel SUJ2 (comparative example B10)has a rolling contact fatigue life L₁₀ of 7,300×10⁴, and S53C is lessthan half the bearing steel. Although the present invention's examples,formed only of inexpensive alloy component, could not be comparable tobearing steel SUJ2, it is desirable that they have an L₁₀ at leastapproximately twice S53C, i.e., at least 5,000×10⁴. In this regard, thepresent invention's examples A1-A9 all provide at least 5,000×10⁴ and inparticular, A5, A8 and A9 have a life equivalent to that of the bearingsteel.

TABLE 2 Rolling Contact Fatigue Ratio in Rolling Life L₁₀(×10⁴) &Sliding Reduction in Measured Expected Fatigue Life Halfwidth After TypeNo. Value Value (to S53C) Fixed Time Note Present A1 6590 6321 1.9 0.4Invention's A2 6120 6043 1.5 0.5 Examples A3 5361 5224 1.7 0.4 A4 55885338 1.4 0.4 A5 7789 7601 1.8 0.3 A6 6850 6668 1.2 0.5 A7 5450 5538 1.60.5 A8 7210 7510 1.5 0.3 A9 7410 6940 1.4 0.4 Comparative B1 2630 24431.0 1.1 S53C Examples B2 5990 5641 0.8 0.6 B3 5530 5840 0.8 0.6 B4 20052271 0.7 0.8 B5 3300 3079 0.9 1.2 B6 1920 1887 0.3 0.9 B7 900 844 0.51.3 B8 2100 2402 0.9 1.1 B9 4610 4886 0.9 0.9  B10 7300 6698 1.4 0.5SUJ2 *1) Reduction relative to X-ray halfwidth of unused specimen

By contrast, among the comparative examples, B2 and B3 provide at least5,000×10⁴ and the other comparative examples that have a relativelysmall alloy element content provide short life. An expected L₁₀ value,as indicated in table 1, is a value obtained as follows: a measured L₁₀value is subjected to multiple regression analysis with an amount ofchemical component C, Si, Mn as a dependent variable and as a result anexpression for estimation:L=11271 (C)+5796 (Si)+2665 (Mn)−6955  (1)is obtained, and from “L” in the expression the expected L₁₀ value isobtained.

Between L and L₁₀ there is a relationship of L₁₀=L×10⁴ Therefore betweenL₁₀ and a component element's content ratio (wt %) the followingexpression:L ₁₀(×10⁻⁴)=11271 (C)+5796 (Si)+2665 (Mn)−6955  (2)is established.

FIG. 3 represents a relationship between each steel's measured L₁₀ valueand the expression for estimation. The figure indicates that they havean excellent correlation. In other words, the amounts of alloy elementsC, Si and Mn, can be used to estimate L₁₀ with high precision. Not onlythe range of the composition C, Si and Mn of the present invention'sexamples A1-A9 but a configuration that allows expression (2) to providean estimated L₁₀ value of at least 5,000×10⁴ would also ensure longlife.

As they roll and slide the present invention's examples A1-A9 allprovide life longer than S53C (comparative example B1). By contrast, thecomparative examples are all inferior to S53C. When alloy composition isconsidered, long rolling and sliding life also requires C, Si and Mn inbalance. Table 2 at the right hand indicates x-ray diffraction halfwidth obtained after a rolling and sliding test is conducted for a fixedperiod of time (or after rotation 9×10⁵ times under the same conditionsas the rolling and sliding test) to indicate a scale of softeningresistance. It is observed that the present invention's examplesgenerally provide small reduction in half width and are less prone tofatigue as they are affected by slippery.

Thus optimizing the amount of C, Si, Mn provides increased rolling lifeand also allows rolling and sliding life to be increased in stablemanner.

EXAMPLE 2

As shown in Table 3, the present invention's examples employ as theirrespective source materials steels A1-A13 providing a stress intensityfactor range associated with tension associated-fatigue crack extensionthat has a lower limit ΔKth of at least 6.2 MPa√{square root over ( )}m.Furthermore for comparative examples steels B 1-B8 having ΔKth smallerthan 6.2 MPa√m are used as source material. Note that as has beenindicated as a note, comparative example B1 is conventional materialS53C and comparative example B8 is bearing steel SUJ2. Furthermore,A1-A9 each have ΔKth of at least 6.2 MPa√m and contain C, Si, Mn fallingwithin a composition range allowing L₁₀ of at least 5,000×10⁴ inexpression (2).

TABLE 3 Alloy Composition (wt %) Type No. C Si Mn Cr Note Present A10.56 0.82 0.83 * Invention's A2 0.60 0.80 0.60 * Examples A3 0.53 0.620.98 * A4 0.63 0.62 0.60 * A5 0.52 1.16 0.74 * A6 0.64 0.83 0.60 * A70.55 0.81 0.60 * A8 0.58 1.00 0.80 * A9 0.61 0.88 0.72 *  A10 0.53 1.000.31 *  A11 0.55 1.00 0.30 *  A12 0.45 0.83 1.10 *  A13 0.45 0.65 1.40 *Comparative B1 0.53 0.20 0.85 * S53C Examples B2 0.63 0.10 0.58 * B30.55 0.11 0.60 * B4 0.53 0.38 0.25 * B5 0.53 0.20 0.25 * B6 0.55 0.200.75 * B7 0.45 0.80 0.80 * B8 1.00 0.25 0.35 1.5 SUJ2 * 0.2~0.3 wt %thereof contained

(1) Fatigue Crack Extension Test

A three-point bending test is conducted to evaluate lower limit ΔKth ofa stress intensity factor range associated with tensionassociated-fatigue crack extension. Each specimen has its external aswell as internal portions uniformly hardened to eliminate effect ofresidual stress on fatigue crack extension. The test is conducted underthe following conditions:

Dimension of specimen: 80 mm×20 mm×2 mm (having a center provided with aslit and having a previous fatigue crack introduced therein)

FIG. 4 shows how a specimen having the above described geometry (of 80mm×20 mm×2 mm) and three-point bending are employed to repeatedly applya load to conduct the test. The specimen has a one side wire cut to havea slit and having an end previously provided with a fatigue crack. Asshown in FIG. 4, when a load P is exerted on the center of the specimenin the three-point bending arrangement with a distance S betweensupporting points, a nominal bending stress σ₀ is exerted as representedin an expression (3). When a crack has a length α(m), stress intensityfactor K_(I) can be obtained by substituting σ₀ in an expression (4),wherein F_(I)(a/W) is a correction factor. FIG. 5 represents arelationship between a/W and F_(I)(a/W) obtained by finite elementmethod (FEM).σ₀=3SP/(2tW ²)  (3)K _(I) =F _(I)(a/W)·σ₀(πa)^(1/2)  (4)

Note that lower limit ΔKth that has the value of the most restrained,and plain strain condition (i.e., Mode I type) is represented byΔK_(I)th. In other words, ΔK_(I)th represents a lower limit of a stressintensity factor range of plain strain condition. In other words, thepresent stress intensity factor range's lower limit ΔKth is a lowerlimit of a stress intensity factor range for plain strain condition. Inthe above description it is simply indicated without a symbol indicatingthat it is of mode I (or plain strain condition). In the followingdescription, the indication “I type” may occasionally be omitted and alower limit of a stress intensity factor range for plain straincondition may occasionally be indicated by ΔKth. The above describedspecimen receives a load exerted by a method of applying the load, asdescribed below:

method of applying a load: load control

load frequency: 8 Hz

stress ratio: 0.5

(2) Fatigue Test

To evaluate fatigue strength, an induction hardened ring specimen isused to conduct a fatigue test. The specimen is induction hardened toallow a hardened layer to have a depth of approximately 2 mm. Four suchspecimens are used and evaluated by average life. The test is conductedunder the following conditions:

dimension of specimen: outer diameter of 60 mm×inner diameter of 45mm×width of 15 mm

load: 9.5 kN

rate of rotation: 8,000 rpm

(3) Rolling Contact Fatigue Test

A rolling contact fatigue test is conducted under the same conditions asexample 1. More specifically, an induction hardened cylindrical specimenis used to conduct the test. The specimen is induction hardened to allowa hardened layer to have a depth of approximately 2 mm. 15 suchspecimens are used and their rolling lives L₁₀ (10% life) are evaluated.

(4) Rolling and Sliding Fatigue Test

A rolling and sliding fatigue test is conducted under the sameconditions as example 1. More specifically, a 2-cylinder test isconducted to conduct the test and a specimen is induction hardened toallow a hardened layer to have a depth of approximately 2 mm. Two suchspecimens are used and evaluated by average life.

(5) Rolling Test With Insufficient Lubrication

When lubricant oil is insufficiently supplied, oil film can partiallybreak and a surface can generate heat and thus crack. To simulate this,the rolling and sliding fatigue test described in item (4) is conducted,although lubricant oil is applied when a specimen starts to roll, andthereafter it is rolled without the lubricant oil further supplied. Twosuch specimens are employed and evaluated by average life.

(6) Test Result

Table 4 indicates the results of the fatigue crack extension test, thefatigue test, the rolling contact fatigue test, the rolling and slidingfatigue test, and the rolling test with insufficient lubrication.

TABLE 4 Ratio in Reduction Ratio of Rolling Contact Fatigue Rolling & inFatigue Life L₁₀(×10⁴) Sliding Halfwidth ΔK_(th) Life Measured ExpectedFatigue Life After Type No. (MPa√m) (to S53C) Value Value (to S53C)Fixed Time Note Present A1 6.5 1.7 6590 6321 1.9 1.7 Invention's A2 6.31.9 6120 6043 1.5 1.6 Examples A3 6.6 1.6 5361 5224 1.7 1.3 A4 6.2 2.05588 5338 1.4 1.3 A5 6.4 1.6 7789 7601 1.8 1.5 A6 6.3 2.1 6850 6668 1.21.4 A7 6.2 1.6 5450 5538 1.6 1.4 A8 6.7 2.2 7210 7510 1.5 1.8 A9 6.5 2.17410 6940 1.4 1.6  A10 6.2 1.7 5990 5641 0.8 1.3  A11 6.3 1.8 5530 58400.8 1.4  A12 6.4 1.5 5610 5859 1.0 1.3  A13 6.3 1.5 5720 5615 1.0 1.3Comparative B1 6.0 1.0 2630 2443 1.0 1.0 S53C Examples B2 5.8 0.7 20052271 0.7 0.7 B3 5.8 0.6 3300 3079 0.9 0.5 B4 5.6 0.4 1920 1887 0.3 1.0B5 5.4 0.2 900 844 0.5 0.9 B6 5.7 0.5 2100 2402 0.9 1.1 B7 5.8 0.9 46104886 0.9 1.2 B8 5.0 not tested 7300 6698 1.4 not tested SUJ2

FIG. 6 illustrates how ΔK_(I)th is obtained in the fatigue crackextension test. FIG. 6 represents a relationship between a crackextension rate da/dN and a stress intensity factor range ΔK_(I) of eachof the present invention's example A1 and comparative example B8 (SUJ2).A stress intensity factor range has lower limit ΔK_(I)th, which is astress intensity factor indicated when applying a load no longer extendsa crack. From the FIG. 6 plot, the present invention's example A1provides ΔK_(I)th of 6.5 Mpa√m and comparative example B8 providesΔK_(I)th of 5.0 Mpa√m.

The fatigue crack extension test reveals that the present invention'sexamples A1-A13 all provide ΔKth of at least 6.2 Mpa√m, whereascomparative examples B1-B8 provide ΔKth of at most 6.0 Mpa√m, which isprovided by B1 (S53C), and the other comparative examples providesmaller ΔKth.

The fatigue test reveals that the present invention's examples A1-A13all provide a fatigue life at least 1.5 times comparative example B1(S53C). Comparative examples B2-B7 are all inferior to comparativeexample B1 (S53C).

As measured, conventional medium carbon steel S53C (comparative exampleB1) has a rolling contact fatigue life L₁₀ of 2,630×10⁴ and bearingsteel SUJ2 (comparative example B10) has a rolling contact fatigue lifeL₁₀ of 7,300×10⁴, and S53C is less than half the bearing steel. Althoughthe present invention's examples, formed only of inexpensive alloycomponent, could not be comparable to bearing steel SUJ2, it isdesirable that they have an L₁₀ at least approximately twice S53C, i.e.,at least 5,000×10⁴. In this regard, the present invention's examplesA1-A9 all provide at least 5,000×10⁴ and in particular, A5, A8 and A9have a life equivalent to that of the bearing steel. By contrast,comparative examples B2-B7all provide short life of less than 5,000×10⁴.An expected L₁₀ value, as indicated in table 3, is a value obtained asfollows: a measured L₁₀ value is subjected to multiple regressionanalysis with an amount of chemical component C, Si, Mn as a dependentvariable and as a result expression (2) is obtained and therefrom theexpected L₁₀ value is obtained.

FIG. 7 represents a relationship between measured and expected L₁₀values. The figure indicates that they have an excellent correlation. Inother words, the amounts of alloy elements C, Si and Mn, can be used toestimate L₁₀ with high precision. Not only the numerical range of C, Siand Mn of the present invention's examples A1-A13 but a configurationthat allows expression (2) to provide an estimated L₁₀ value of at least5,000×10⁴ would also ensure long life.

As they roll and slide the present invention's examples A1-A9 allprovide life longer than S53C (comparative example B1). In contrast,A10-A13 have a rolling and sliding life equivalent to or slightlyshorter than S53C and the comparative examples except B8 (SUJ2) are allinferior to S53C. When alloy composition is considered, any example,either the present invention's or the comparative examples, thatcontains a large amount of Mn has a tendency to provide long rolling andsliding life.

The rolling test with small lubrication reveals that the presentinvention's examples A1-A13 all superior to S53C (comparative exampleB1). Among the comparative examples, B6 and B7, containing two of C, Siand Mn in large amount, provide a relatively long life, however they donot have such a long life as provided under the above describedconditions for the rolling and sliding fatigue test.

Thus to allow medium carbon steel based material to provide longerfatigue life, longer rolling contact fatigue life, longer rolling andsliding fatigue life, and longer life despite insufficient lubrication,a ΔKth of at least 6.2 Mpa√m is first of all necessary. Furthermore, toensure that ΔKth, C, Si and Mn can be adjusted in amount to improveother performance in reliability.

The present induction hardened rolling component can have a rolling partor induction hardened portion having a surface resistant to cracking andits propagation and can also have a rolling life comparable to that ofbearing steel as the component rolls as well as it slides and/orrepeated tensile stress strength other than rolling is superimposed, andwhen the above rolling component is used in a power transmissioncomponent, the power transmission component can have both increasedstrength and increase life.

Hereinafter the above described examples as well as the presentinvention's embodiment and its function and effect, and alloy element'sfunction and effect in particular, will comprehensively be described.

The present rolling component in one embodiment can be formed of a steelmaterial having an alloy composition range, as described hereinafter,allowing the steel's induction hardened portion to have the abovedescribed ΔKth that has a value of at least 6.2 Mpa√m to ensure furtherincreased life. More specifically, a rolling component may be configuredwithin a composition containing 0.5-0.7 wt % of C, 0.6-1.2 wt % of Siand 0.6-1.5 wt % of Mn, and the remainder formed of Fe and anunavoidable impurity.

The carbon content of 0.5-0.7 wt % is adopted because it allowsinduction hardening to ensure at least a level of hardness and it cancoexist with a fixed amount of Si, Mn and the like to ensure a rollingcontact fatigue life under large load. To achieve this, a carbon contentof 0.5 wt % or more is required. To achieve further increased hardnessand further increased rolling contact fatigue life, it is desirable that0.55 wt % or more of carbon be contained.

Carbon forms carbide, and to obtain constant hardness, larger amounts ofcarbon are better introduced. Excessively large amounts of carbon,however, result in source material having excessively increased hardnessand poor workability. Furthermore, it requires soaking to preventcomponent segregation, carbide spherodization, and/or similar specialheat treatment, which invites increased cost, and accordingly, 0.7 wt %is set as an upper limit. Furthermore, to ensure the Mn's effect asdescribed above and in addition thereto reduce a detriment associatedwith component segregation, a carbon content set to at most 0.65 wt % isdesirable.

Si is an element reinforcing steel's base metal to provide increasedrolling life. Containing 0.6 wt % or more of Si allows the steel to beresistant to softening despite exposure to high temperature to retardstructural variation, cracking and/or the like attributed to large loadrepeatedly exerted. Accordingly, Si has a lower limit set to be 0.6 wt%. To obtain further increased resistance to softening despite exposureto at high temperature, 0.7 wt % or more is desirable.

On the other hand, increasing the amount of Si does not contribute toincrease source material in hardness as Mn does, as described later, andSi exceeding 1.2 wt % impairs cold workability and hot workability.Accordingly, 1.2 wt % is set as an upper limit. Furthermore, to preventa surface from having a property impaired by internal oxidation of Si ata surface during hot working, Si is desirably set to be 1.1 wt % orless. Furthermore to alleviate surface decarburization, 1.0 wt % or lessis desirable.

Containing 0.6 wt % or more of Mn improves steel material'squenchability. Furthermore, the steel is toughened by solid solution ofMn and retained austenite beneficial for rolling contact fatigue life isincreased. Furthermore, Mn is also effective in improving lower limitΔKth of a stress intensity factor range associated with tensionassociated-fatigue crack extension. Accordingly, 0.6 wt % or more of Mnis contained. To further increase retained austenite to obtain furtherincreased rolling life, 0.7 wt % or more of Mn is desirably contained.

On the other hand, Mn, as well as Si, acts to reinforce source materialand in addition thereto also enters carbide and acts to increase thecarbide's hardness. Containing 1.5 wt % or more of Mn excessivelyincreases source material in hardness and impairs it in workability andgrindability. Accordingly, Mn is contained in an amount having an upperlimit of 1.5%. To provide reduced level of component segregation and forexample to reduce the cost for soaking, Mn set to 1.25 wt % or less isdesirable. Furthermore, if a cast ingot in production has a large size,more significant component segregation is caused. Accordingly, Mn set to1.0 wt % or less is further desirable.

If an electric furnace is employed to provide ingot steel, scraps areemployed as main source material, and impurities contained in the scrapsare introduced into the steel. For example, 0.3 wt % or less of Cr, 0.3wt % or less of copper, and other impurities are introduced from thescraps into the steel. Such impurities introduced from source materialfor producing steel are considered as unavoidably contained impurities.In other words, if a rolling component contains such impurity therolling component corresponds to the present rolling component.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

INDUSTRIAL APPLICABILITY

In accordance with the present invention a rolling component can beformed of steel material including medium carbon steel having acomposition adjusted to provide a rolling contact fatigue life increasedto be comparable to that of bearing steel containing an expensive alloyelement and also provide improved characteristic againstsurface-cracking. As a result the present rolling component and powertransmission component employing the rolling component are expected tobe used in automobiles and the like pursuing energy conservation andminiaturization at power transmission systems in which there is slipperyand repeated tensile stress other than rolling is superimposed.

1. A rolling component formed of a steel having an induction hardened portion having a stress intensity factor range associated with tension associated-fatigue crack extension that has a lower limit ΔKth of 6.2 Mpa√m.
 2. The rolling component of claim 1, wherein said steel has a composition containing 0.5-0.7 wt % of C, 0.6-1.2 wt % of Si, 0.6-1.5 wt % of Mn, and a remainder formed of Fe and unavoidable impurity.
 3. The rolling component of claim 1, wherein said steel is steel material having C, Si and Mn adjusted in content, as represented in wt %, to satisfy L ≧5,000 in the following expression: L=11271(C)+5796(Si)+2665(Mn)−6955   (1).
 4. A power transmission component comprising the rolling component of claim
 1. 5. The power transmission component of claim 4, corresponding to a hub joint including a wheel bearing and a constant velocity joint united together.
 6. The power transmission component of claim 4, wherein said rolling component slides as it rolls. 